The invention relates generally to screens of the type commonly used for water intakes for supplying industrial plants or equipment, and more particularly, though not exclusively, to revolving screens such as drum screens and band screens.
Such water intakes are usually installed along the bank of a waterway or at the sea coast. The water inevitably entrains, in variable amounts, numerous foreign bodies, manmade waste, industrial refuse, and debris and organisms of natural origin such as leaves, grass, branches, fish, and algae.
Before use of such water in industrial plants or a piece equipment the water is screened, first by passing through a bar screen and then through a revolving screen. The revolving screen may, for example, be a band screen comprising a screen band including a series of articulated screen panels arranged in an endless loop or a drum screen which may also comprise a series of screen panels which together form the cylindrical sidewall of the drum.
Revolving screens may be disposed transversely with respect to the direction of the flow of the water in the sluice in which case the generatrices of the screen band or screen drum are substantially perpendicular to the direction of flow or they may be disposed parallel to the direction of flow of the water in the sluice in which case the generatrices of the screen band are substantially parallel to the direction of flow.
In so-called flow-through band screens the band screen disposed transversely with respect to the direction of the flow of water in the sluice extends across the entire width of the sluice and the water passes successively through an upstream run and a downstream run of the screen before continuing on toward the plant or piece of equipment in which it is to be used.
Debris held back by the screen is deposited on the upstream or exterior surface of the upstream run of screen.
In an other type of band screens called a dual-flow band screen the water to be screened simultaneously passes through both runs of the screen and depending on the selected direction of the flow of water through the screen the debris held back is deposited on the exterior or interior surface of the screen.
The advantages of the different types of revolving screen and various arrangements threof are known to those skilled in the art and need not be developed herein.
Whatever the type of revolving screen, the screen element becomes clogged during its immersion in the sluice and the rotation of the screen brings the clogged elements in line with a cleaning or washing device disposed at the top, emersed part of the screen and typically comprises at least one spray nozzle disposed on a first side of the screen element opposite the side on which the debris clogging the same is deposited and adapted to spray a jet of liquid toward the screen element, in practice in the form or a flat spray or jet. A debris recovery member, usually a channel, is associated with the cleaning or washing device and is adapted to collect the debris which the sprayed liquid has removed from the screen element. There is inevitably a gap between the screen element and the debris recovery channel, especially in band screens, to compensate for the sway of the screen band in operation.
Such cleaning or washing devices are adapted to remove debris from the rising or descending run of the screen element when the cleaning or washing device is disposed along the emersed part of the rising run of the screen element.
As soon as it is touched by the lower generatrice or edge of the liquid spray, it falls, under the force of gravity, between the screen element and the edge of the debris recovery channel. In particular when the debris deposited on the screen element forms a layer which is unstuck in a single mass or comprises large leaves, filaments or especially algae, the liquid spray entrains the debris against the side or edge of the recovery channel to which it becomes attached and builds up dangerously.
If the cleaning or washing device is disposed along the descending run or side of the screen element, the detached debris falls on the liquid spray and is carried into the debris recovery channel. However, debris which comprises relatively heavy pieces, debris having a large surface area (plastic film or sheets), or comprising long strips such as algae which escapes the liquid spray falls and/or attaches itself to the edge or lip of the debris recovery channel. The consequent build-up of debris on the edge build-up is all the more dangerous as the displacement of the screen panels downwards past the debris recovery channel increases the stuffing up of the gap between the screen element and the debris recovery channel.
Now, particularly when the water intake has to handle water carrying large amounts of debris, for example, algae, the removal of the debris must be effected continuously and any build-up anywhere along the debris recovery device, and in particular in the debris recovery channel, may jam the screen, putting it out of order. Likewise, the stuffing up of the gap between the edge or side of the debris recovery channel and the filter element may seriously damage the device and cause the stoppage of the screen.